Apparatus and methods for generating a selection signal to perform an arbitration in a single cycle between multiple signal inputs having respective data to send

ABSTRACT

An interconnect within an integrated circuit provides arbitration to select one of a plurality of signal inputs for connection to a signal output. The arbitration applied uses a first arbitration parameter value, in the form of a time stamp value, and, if two or more signal inputs share such a time stamp value, then uses a second arbitration parameter, in the form of a least recently granted value. The time increment applied to the time stamp value associated with each signal input when it is granted access to the signal output is selected to reflect the quality of service to be associated with that signal input. When a comparison is made between time stamp values, the lowest time stamp value is given priority. A large time increment value corresponds to a low priority (quality of service).

This application is a continuation of U.S. patent application Ser. No.13/940,915, filed 12 Jul. 2013, entitled “SINGLE CYCLE ARBITRATIONWITHIN AN INTERCONNECT”, which is a continuation-in-part of U.S. patentapplication Ser. No. 13/438,920, filed 4 Apr. 2012, entitled “CROSSBARCIRCUITRY FOR APPLYING AN ADAPTIVE PRIORITY SCHEME AND METHOD OFOPERATION OF SUCH CROSSBAR CIRCUITRY”, which is a continuation-in-partof U.S. patent application Ser. No. 12/926,462, filed 18 Nov. 2010,entitled “CROSSBAR CIRCUITRY FOR APPLYING AN ADAPTIVE PRIORITY SCHEMEAND METHOD OF OPERATION OF SUCH CROSSBAR CIRCUITRY”, which is acontinuation-in-part of U.S. patent application Ser. No. 12/458,511,filed 14 Jul. 2009, entitled “CROSSBAR CIRCUITRY AND METHOD OF OPERATIONOF SUCH CROSSBAR CIRCUITRY”, which is a continuation-in-part of U.S.patent application Ser. No. 12/379,191, filed 13 Feb. 2009, entitled“CROSSBAR CIRCUITRY AND METHOD OF OPERATION OF SUCH CROSSBAR CIRCUITRY”.The contents of all of these applications are incorporated herein intheir entirety by reference.

BACKGROUND

This invention relates to the field of data processing systems. Moreparticularly, this invention relates to the field of interconnectcircuitry for providing communication between a selected one of aplurality of signal inputs and a signal output by applying anarbitration policy.

It is known to provide interconnect circuitry for providing acommunication path for data between a selected one of a plurality ofsignal inputs and a single output. Such multiplexing circuitry may applyan arbitration policy so that certain signal inputs are given priorityin securing access to the signal output. A challenge within suchinterconnect circuitry is to reduce the time taken for the arbitration(thereby reducing the number of cycles taken for arbitration orpermitting use of a higher clock frequency) while ensuring fairnessbetween the signal inputs, e.g. where more than one parameter controlsarbitration.

SUMMARY

Viewed from one aspect the present invention provides an interconnectcircuitry for providing a communication path for data between a selectedone of N signal inputs and a signal output, where N is an integer of twoor more, said interconnect circuitry comprising:

multiplexing circuitry configured to select one of said N signal inputsas a selected signal input in dependence upon a selection signal and toconnect said selected signal input to said signal output to send givendata; and

arbitration circuitry configured to generate said selection signal so asto perform an arbitration in a single cycle between a plurality of saidN signal inputs having respective data to send, wherein

said arbitration is performed in dependence upon:

(i) respective values of a first arbitration parameter associated witheach of said plurality of said N signal inputs; and

(ii) when two or more of said plurality of said N signal inputs have acommon value of said first arbitration parameter, a second arbitrationparameter associated with each of said two or more of plurality of saidN signal inputs, said second arbitration parameter having a differentvalue for each of said two or more of said plurality of said N signalinputs.

The present technique performs arbitration based upon a firstarbitration parameter and a second arbitration parameter. When two ormore of the signal inputs have a common value for the first arbitrationparameter, then the second arbitration parameter is used to resolvebetween the signal inputs which otherwise have the same priority level,as the second arbitration parameter is arranged such that it has adifferent value for at least each of the two or more signal inputs whichtie in their first arbitration parameter. In some embodiments the secondarbitration parameter may be arranged to have a unique value for eachsignal input irrespective of whether or not any ties occur with thefirst arbitration parameter.

The plurality of signal inputs may be connected to respective datasource circuits and the signal output may be coupled to a datadestination circuit. The data source circuits and the destinationcircuits may be formed together with the interconnect circuitry on asingle integrated circuit. Intra-circuit communication within anintegrated circuit is a significant processing bottleneck as integratedcircuits increase in the sophistication of their design and the numberof different functional blocks provided within an integrated circuit,e.g. in system-on-chip integrated circuits.

While it will be appreciated that the present technique may be used insituations where there is only a single signal output, the presenttechnique is well suited to embodiments in which there are a pluralityof signal outputs, with access to each of the signal outputs beingseparately arbitrated in accordance with its own arbitration priorities(which may or may not be the same between different signal outputs). Thenumber of inputs may be the same as the number of outputs providing asymmetrical arrangement, although other arrangements are possible wherethe number of inputs is different to the number of outputs.

The first arbitration parameter may be arranged to a have a valuerepresenting a quality of service level associated with data sent from acorresponding input. In this way, the arbitration using the firstarbitration parameter may be arranged to provide access to the signaloutput in a manner which is matched to a desired quality of servicelevel associated with respective signal inputs.

Various different forms of first arbitration parameter are possible,e.g. it would be possible to form a first arbitration parameter basedupon the number of packets of data a given input has been able to sendduring a certain period. One form of first arbitration parameter thatmay be used to provide arbitration representing a quality of servicelevel while fairly apportioning access to the signal output between thedifferent signal inputs is one in which the first arbitration parameteris a time stamp value and the time stamp value for the selected signalinput is updated when that selected signal input sends data. Associatinga time stamp with each signal input when it sends data may be used as away of assisting in fairly partitioning access to the signal outputbetween a plurality of signal inputs. The time stamp may indicate whendata was sent, or when the signal input concerned is expecting to nextbe able to send data, in accordance with its fair allocation of thebandwidth associated with the signal output.

Updating the time stamp value by a time increment value that varies independence upon a quality of service level associated with the selectedsignal input is a way of partitioning the available bandwidth of thesignal output between the different signal inputs. The incremented timestamp value can represent when a given signal input can next fairlyexpect to have access to the signal output. A high priority signal inputwould have a small time increment applied so that it will relativelyquickly re-qualify to be able to gain access to the signal output.Conversely, a low priority signal input will have a relatively largetime increment applied such that a relatively long period will expirebefore it is eligible for access to the signal output. It will beappreciated that when there is no contention for access to the signaloutput, then any of the signal inputs which has data to send may begranted access to the signal output irrespective of its associated timestamp value.

The arbitration circuitry may be configured to compare time stamp valuesassociated with the signal inputs which have data to send and arecontending for access to the signal output. The arbitration circuit caneliminate from the possibility of selection as the selected signal inputany of the plurality of inputs having data to send that has a highertime stamp value than one or more of the other of the plurality ofsignal inputs that have data to send. Thus, if a single signal input hasthe lowest time stamp value, then all the other signal inputs will beeliminated. The time stamps reflects the history of access. A low timestamp value indicates that an associated signal input has not receivedits fair share of access to the signal output relative to the othersignal inputs. If two or more signal inputs share the lowest time stampvalue, then all the other remaining signal inputs will be eliminated.

In order to address size constraints associated with the storage andmanipulation of time stamp values and the eventual overflow of timestamp values, the arbitration circuitry may be configured such that whenat least one of the time stamp values associated with one of the signalinputs reaches a threshold level, then all of the time stamp values aredivided by two (right shifted by one bit position). While such anapproach loses some resolution in the differentiation between the timestamp values that are stored, the coarse level relative ordering of thetime stamp values, above the level of lost resolution, is maintained.

The second arbitration parameter associated with each of the signalinputs can have a variety of different forms providing that the secondarbitration parameter values are different for at least those signalinputs which may share a common first arbitration parameter value. Onesuch form of second arbitration parameter which provides fairness andguarantees a resolution to determining which signal input should beselected is one in which the second arbitration represents a relativeorder in which the signal inputs were previously selected to serve asthe selected signal input. For example, the second arbitration parametermay represent the relative order in the form of a least recently granted(LRG) parameter with the signal that has the longest duration since itwas previously granted access to the signal output having the highestpriority second arbitration parameter. Other forms of second arbitrationparameter may also be used, e.g. the second arbitration parameter may beallocated on the basis of a static priority, a round robin algorithm,etc.

The first arbitration parameter and the second arbitration parameter maybe concatenated to form an at least logically combined arbitrationparameter with the second arbitration parameter controlling a leastsignificant bit portion of the combined arbitration parameter.Concatenating the two arbitration parameters in this way simplifies andspeeds up their comparison whilst maintaining their relativesignificance, i.e. the hierarchy by which they act to control selectionof a particular signal input as the selected signal input.

The comparison of the combined arbitration parameter may be speeded upwhen the arbitration circuitry comprises thermometer coding circuitrythat serves to thermometer code the logically combined arbitrationparameter to generate a thermometer coded arbitration parameter.Thermometer coded arbitration parameters are well suited to parallelcomparison to one another in a manner in which the highest priority suchthermometer coded arbitration parameter may be identified within asingle cycle. The different portions of the combined arbitrationparameter may in practice be stored in different structures, but acttogether to provide an overall thermometer coding.

The comparison circuitry used to perform the comparison betweenthermometer coded arbitration parameters may comprise a plurality ofsignal lines each pre-charged to a determined signal level with thesesignal lines then each being selectively discharged in dependence uponthe thermometer coded arbitration parameters.

The plurality of signal lines which are selected to be discharged may bedivided into groups such that, if there are N signal inputs betweenwhich a selection is to be made, then are 2 ^(X) groups of signal lineseach associated with a different value of the first arbitrationparameter, where X is the bit length of the first arbitration parameter,when this has been subject to thermometer coding. The comparisoncircuitry may be configured to discharge all the signal lines within agiven group of signal lines if any of the N signal inputs has a firstarbitration parameter indicative of a higher priority than a firstarbitration parameter associated with that given group. Thus, a higherpriority signal input will discharge the signal lines associated withlower priority signal inputs in a manner which represents the comparisonof priority levels that it is desired to perform.

Each of the groups of signal lines may contain N signal lines withdifferent signal lines within each group of signal lines correspondingto different respective unique values of the second arbitrationparameter. The comparison circuitry may be configured to discharge thedifferent signal lines within a threshold group, i.e. a group that isassociated with a highest priority first arbitration parameter, independence upon the second arbitration parameter associated with the Nsignal inputs such that a single signal line within the threshold groupwill remain charged thereby identifying which of the N signal inputs isto be selected as the selected signal output. Thus, the firstarbitration parameter may be considered to control the discharging ofthe groups as a whole which correspond to lower priority values of thefirst arbitration parameter, with discharge within the threshold groupbeing performed in dependence upon the second arbitration parameter suchthat a single signal input may be selected in accordance with thatsecond arbitration parameter if two or more signals share a value of thefirst arbitration parameter.

The signal lines which are used to provide the above describedarbitration may be conveniently reused to also communicate the databetween the signal inputs and the signal output once the arbitration hasresolved.

Viewed from another aspect the present invention provides interconnectcircuitry for providing a communication path for data between a selectedone of N signal inputs and a signal output, where N is an integer of twoor more, said interconnect circuitry comprising:

multiplexing means for selecting one of said N signal inputs as aselected signal input in dependence upon a selection signal and toconnect said selected signal input to said signal output to send givendata; and

arbitration means for generating said selection signal so as to performan arbitration in a single cycle between a plurality of said N signalinputs having respective data to send, wherein

said arbitration is performed in dependence upon:

(i) respective values of a first arbitration parameter associated witheach of said plurality of said N signal inputs; and

(ii) when two or more of said plurality of said N signal inputs have acommon value of said first arbitration parameter, a second arbitrationparameter associated with each of said two or more of plurality of saidN signal inputs, said second arbitration parameter having a differentvalue for each of said two or more of said plurality of said N signalinputs.

Viewed from another aspect the present invention provides a method ofproviding a communication path for data between a selected one of Nsignal inputs and a signal output, where N is an integer of two or more,said method comprising the steps of:

selecting one of said N signal inputs as a selected signal input independence upon a selection signal and to connect said selected signalinput to said signal output to send given data; and

generating said selection signal so as to perform an arbitration in asingle cycle between a plurality of said N signal inputs havingrespective data to send, wherein

said arbitration is performed in dependence upon:

(i) respective values of a first arbitration parameter associated witheach of said plurality of said N signal inputs; and

(ii) when two or more of said plurality of said N signal inputs have acommon value of said first arbitration parameter, a second arbitrationparameter associated with each of said two or more of plurality of saidN signal inputs, said second arbitration parameter having a differentvalue for each of said two or more of said plurality of said N signalinputs.

The above, and other objects, features and advantages of this inventionwill be apparent from the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an integrated circuit including aswizzle switch interconnect connecting a plurality of data sourcecircuits to a plurality of data destination circuits;

FIG. 2 schematically illustrates a first arbitration parameter and asecond arbitration parameter being concatenated to form a combinedarbitration parameter, which is itself then subject to thermometercoding to form a thermometer coded arbitration parameter;

FIG. 3 is a flow diagram schematically illustrating selecting which datato send and updating the arbitration values;

FIG. 4 is a flow diagram schematically illustrating arbitration; and

FIG. 5 schematically illustrates arbitration circuitry provided in theform of signal lines which are precharged and then selectivelydischarged in dependence upon a first arbitration parameter and a secondarbitration parameter.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 schematically illustrates an integrated circuit 2 including aplurality of data source circuits 4 connected via interconnect circuitry6 to a plurality of data destination circuits 8. The form of theinterconnect circuitry 6 may be that of a swizzle switch interconnect asdescribed in co-pending U.S. patent application Ser. No. 13/438,920 thecontent of which is incorporated herein in its entirety by reference.

The swizzle switch interconnect 6 provides the ability to connect any ofthe data source circuits 4 to any of the data destination circuits 8. Inthis example, there are four data source circuits 4 and the same numberof data destination circuits 8. However, these numbers could bedifferent. Also, it will be appreciated that the data source circuits 4and the data destination circuits 8 are illustrated as distinctentities, whereas in practice one or more of these may be a commonentity, e.g. a general purpose processor connected to the interconnectcircuitry 6 may serve as both a data source and a data destination.

As described in the above referenced co-pending application, theinterconnect circuitry 6 is provided with signal lines which may beprecharged and then selectively discharged to perform the tasks ofarbitrating between the sources and destinations as well as, in asubsequent cycle, transferring data values between the sources and thedestinations. Any of the data source circuits 4 may be connected to anyof the data destination circuits 8 and accordingly at the intersectionsillustrated in FIG. 1 there is provided multiplexing circuitry. A columnof the multiplexing circuitry indicated in FIG. 2 serves to select oneof the N data source circuits 4 for connection to the data destinationcircuit 8 at the foot of that column. The arbitration circuitry, whichis present within the interconnect circuitry 6, is provided in layersbelow the layers which provide the signal lines which are precharged andthen selectively discharged. Thus, while the figures herein show thearbitration circuitry and the wires (signal lines) side-by-side, inpractice the arbitration circuitry will be in a layer of an integratedcircuit below the metal layer providing the wires and so will notconsume any additional circuit area. This arbitration circuitry will bedescribed later.

FIG. 2 schematically illustrates a p-bit first arbitration parameter 10and a q-bit second arbitration parameter 12. The first arbitrationparameter 10 may take the form of a time stamp value allocated inaccordance with a virtual clock value which is maintained and updated inrespect of the arbitration performed for each of the signal outputs, andwhich is sampled and associated with respective signal inputs triggeredby those signal inputs being permitted to send a data value to thesignal output. The second arbitration parameter 12 may be in the form ofa value representing which of the signal inputs has least recently beengranted access to the signal output. Thus, each time one of the signalinputs is granted access, then all of the least recently granted valuesassociated with the other signal inputs are updated to reflect the newrelative ordering. Use of such a second arbitration parameterrepresenting least recently granted status provides a second arbitrationparameter in which each of the signal inputs has a different secondarbitration parameter value. This ensures that the second arbitrationparameter value can be used to resolve arbitration between signal inputswhen those signal inputs share a first arbitration parameter value.

As illustrated in FIG. 2, the first arbitration parameter 10 and thesecond arbitration parameter 12 may be concatenated to perform acombined arbitration parameter 14 with the second arbitration parameterforming the least significant portion of this combined arbitrationparameter 14. Combining the first arbitration parameter 10 and thesecond arbitration parameter 12 in this way ensures that the secondarbitration parameter 12 will only be effective in discriminatingbetween priority levels when the higher order bit portion correspondingto highest current value of the first arbitration parameter 10 is commonto two or more different signal inputs.

In order to speed up the comparison between arbitration parametervalues, the combined arbitration parameter value 14 is subject tothermometer coding to produce a thermometer coded arbitration parameter16 that has 2^(p+q). Such thermometer coded arbitration parameter valuesare better suited to parallel comparison in a single cycle permittingrapid determination of which signal input is to be given access to thesignal output.

FIG. 3 is a flow diagram schematically illustrating the control appliedto the sending of data and the updating of arbitration values. At step18 processing waits until at least one of the signal inputs has a datavalue to be communicated to the signal output and the data channelprovided by the swizzle switch interconnect is free (available). Step 20determines whether there are more than one signal input with data readyto send. If there are more than one signal input with data ready tosend, then step 22 performs arbitration (described below) between thesesignal inputs to select a single one as the selected signal input whichis to be permitted to send its data. At step 24 a selection signal isgenerated for the selected signal input which controls the multiplexingcircuitry illustrated in FIG. 1 to select the signal input and permit itto pass its data value (or burst of data values if this is permitted bya particular embodiment) to the signal data output. This transfer ofdata is performed at step 26. At step 28 the time stamp of the selectedsignal input is incremented by a time increment which represents aquality of service level associated with the selected signal input. Asignal input with a high quality of service level (a high level ofbandwidth to be allocated to it) is associated with a relatively smalltime stamp increment. Conversely, a signal input with a relatively lowpriority and low quality of service level (corresponding to relativelylow bandwidth allocation) has a relatively high time stamp incrementapplied to it. When comparing the time stamp values associated with thesignal inputs, then the lowest time stamp value is given priority. Thetime stamp value associated with a single input can be considered asrepresenting the time at which that signal can reasonably expect to begiven access to the signal output given its priority level andallocation of the bandwidth when there is contention between signalinputs for access to the signal output. If there is no contention, thenany signal input of a data to send can send that data and update itstime stamp value. When two or more signal inputs have data to send, thenthe time stamp values of those signal inputs serve as the firstarbitration parameter and may be compared to determine which of thosesignal inputs should be given access to the signal output.

Following the increment to the time stamp applied in step 28, processingproceeds to step 30 where the least recently granted values associatedwith each of the signal inputs are updated to reflect the granting tothe signal input, which was selected by the selection signal at step 24,access to the signal output. All of the least recently granted valueswill be updated to reflect the new relative ordering in which they havebeen granted access to the signal output. The least recently grantedvalues serve as the second arbitration parameter and are all different.

FIG. 4 is a flow diagram schematically illustrating the arbitrationperformed at step 22. Step 32 waits until arbitration is required. Step34 then compares time stamp values. Step 36 determines if there is morethan one signal input with the lowest time stamp value. If thedetermination at step 36 is that there is only one signal input with thelowest time stamp value, then this signal input is selected at step 38and processing returns to step 32. If the determination at step 36 isthat there is more than one signal input with the lowest time stampvalue, then processing proceeds to step 40 where the least recentlygranted values associated with those signal inputs with the lowest timestamp values are compared. Thus, the second arbitration parameter takeseffect when signal inputs share a common value for the first arbitrationparameter (lowest time stamp value). Step 42 selects for communicationwith the signal output during this single cycle evaluation the signalinput with the highest priority least recently granted value. The leastrecently granted value with the highest priority is that which indicatesthat the signal input concerned has not been granted access to thesignal output for the longest time.

FIG. 5 schematically illustrates arbitration circuitry 44 forcontrolling access to one of the signal outputs of the interconnectcircuitry 6 illustrated in FIG. 1. The most significant bits of avirtual clock count value serve as a first arbitration parameter and aresubject to thermometer coding by thermometer coding circuitry 46 toproduce a thermometer coded value having one bit position associatedwith a respective group of signal lines 48 passing through theinterconnect 6. The sense amplifier enable latch 50, which is driven bythe portion of the arbitration circuitry 44 illustrated in FIG. 5, whenactive serves to select signal input 0 to have access to the signaloutput when the first arbitration value and the second arbitration valueare associated with signal input 0 win the arbitration as they have thehighest priority (associated with their quality of service). Thethermometer coded arbitration value generated by the thermometer codingcircuitry 46 is the arbitration parameter associated with signal input 0at that point in time. A first portion of this arbitration parametervalue is shown as comprising bits (0) to (15) in FIG. 5 with each ofthese bits controlling switching of a multiplexer 52 that eitherdischarges none of the signal lines 48 within a group, all of the signallines 48 within a group or some of the signal lines in dependence upon asecond arbitration parameter given by a thermometer coded leastsignificant portion representing the least recently granted value andstored within register 54.

As illustrated in FIG. 5 when the time stamp value reaches a level atwhich it is close to saturation, or at least one of the time stampvalues is close to saturation, then a right shift by half its bit lengthis applied to the thermometer coded arbitration values stored within thethermometer coding circuitry 46, the register 60 and also the leastsignificant bits of the virtual clock counter. The comparison circuitryis provided by gates 56 which selectively discharge the prechargedsignal lines in dependence upon the thermometer coded arbitration value.It will be appreciated that FIG. 5 illustrates the selective dischargeof the signal lines 48 in respect of a single signal input, but inpractice selected discharge of the same signal lines 48 is alsoperformed in respect of the other signal inputs, which will have theirown thermometer coded arbitration values. The overall effect is that thecomparison circuitry 56 for a given signal input will discharge thegroups of signal lines associated with arbitration values of lowerpriority, and will not discharge the groups of signal lines 48associated with arbitration values of a higher priority. If two signalinputs have arbitration values that share a first arbitration valueparameter, then the boundary between the groups of signal lines whichare discharged and not discharged will be the same for those two signalinputs. Accordingly, the partial discharge of signal lines performed bythe multiplexer 52 within the group of signal lines at the boundarybetween the “1” bits and the “0” bits within the thermometer codedarbitration value will selectively discharge those lines in accordancewith their own individual least recently granted values (secondarbitration parameter values) and accordingly a comparison will be madebetween these least recently granted values such that the signal inputwith the highest priority least recently granted value will beidentified. The multiplexer 58, which is controlled by the firstarbitration parameter in binary form stored within register 60, servesto select one of the groups of signal lines which will indicate for thatsignal input with a given second arbitration parameter value whether ornot that signal input is determined as winning any arbitration. Theinputs to the multiplexer 58 for the signal input illustrated areconnected to signal lines 0, 4, 8, . . . 60. The inputs to themultiplexer 58 for the next signal input, namely signal input 1, willcorrespond and will be taken from signal lines 1, 5, 9, . . . 61.

At an overall level, the arbitration circuitry 44 provides a singlecycle arbitration using both a first arbitration parameter valuecorresponding to a time stamp value (which is subject to a timeincrement upon grant in dependent upon associated quality of servicelevel) as well as a second arbitration parameter value (LRG value)provided for the purposes of a tie break resolution in the same cyclethat the first arbitration parameter values are compared. Theprecharging and selective discharging of the signal lines 48 provides amechanism supporting parallel comparison of multiple such arbitrationparameter values within a single cycle.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

We claim:
 1. Arbitration circuitry configured to generate a selectionsignal so as to perform an arbitration in a single cycle between aplurality of N signal inputs having respective data to send, where N isan integer of two or more, wherein said arbitration is performed independence upon: (i) respective values of a first arbitration parameterassociated with each of said plurality of N signal inputs; and (ii) whentwo or more of said plurality of N signal inputs have a common value ofsaid first arbitration parameter, a second arbitration parameterassociated with each of said two or more of plurality of N signalinputs, said second arbitration parameter having a different value foreach of said two or more of said plurality of N signal inputs. 2.Arbitration circuitry as claimed in claim 1, wherein said firstarbitration parameter has a value representing a quality of servicelevel associated with data sent from a corresponding signal input. 3.Arbitration circuitry as claimed in claim 1, wherein said secondarbitration parameter is allocated to each of said N signal inputs torepresent a relative order in which said N signal inputs were previouslyselected as said selected signal input.
 4. Arbitration circuitry asclaimed in claim 1, wherein said first arbitration parameter and saidsecond arbitration parameter are concatenated to form a combinedarbitration parameter with said second arbitration parameter controllinga least significant bit portion of said combined arbitration parameter.5. Arbitration circuitry as claimed in claim 2, wherein said firstarbitration parameter is a time stamp value and said time stamp valuefor a selected signal input selected in dependence on said selectionsignal is updated when said selected signal input sends data. 6.Arbitration circuitry as claimed in claim 4, wherein said arbitrationcircuitry comprises thermometer coding circuitry configured tothermometer code said combined arbitration parameter of each of said Nsignal inputs to generate respective thermometer coded arbitrationparameters.
 7. Arbitration circuitry as claimed in claim 5, wherein saidtime stamp value for said selected input is updated by adding a timeincrement value to said time stamp value for said selected input, saidtime increment value varying in dependence upon a quality of servicelevel associated with said selected signal input.
 8. Arbitrationcircuitry as claimed in claim 5, wherein said arbitration circuitry isconfigured such that when at least one of said time stamp valuesassociated with said N signal inputs reaches a threshold level, then allof said time stamp values are divided by two.
 9. Arbitration circuitryas claimed in claim 6, wherein said arbitration circuitry comprisescomparison circuitry configured to compare said thermometer codedarbitration parameters to identify which of said N signal inputs isselected as said selected signal input.
 10. Arbitration circuitry asclaimed in claim 7, wherein said arbitration circuitry is configured tocompare time stamp values of a plurality of signal inputs having data tosend and to eliminate from selection as said selected signal input anyof said plurality of inputs having data to send that have a higher timestamp that one or more other of said plurality of signal imputs havingdata to send.
 11. Arbitration circuitry as claimed in claim 9, whereinsaid comparison circuitry comprises a plurality of signal lines eachprecharged to predetermined signal level and selectively discharged independence upon said thermometer coded arbitration parameters. highertime stamp value that one or more other of said plurality of signalinputs having data to send.
 12. Arbitration circuitry as claimed inclaim 11, wherein said signal lines are also used to communicate saiddata.
 13. Arbitration circuitry as claimed in claim 10, wherein as saidtime increment value associated with a given signal input increases, arelative priority for access to said signal output by said given signalinput among said N signal inputs decreases.
 14. Arbitration circuitry asclaimed in claim 11, wherein said plurality of signal lines is dividedinto 2^(X) groups of signal lines each associated with a different valueof said first arbitration parameter, where X is the bit length of saidfirst arbitration parameter, said comparison circuitry configured todischarge all signal lines within a given group of signal lines if anyof N signal inputs has a first arbitration parameter indicative of ahigher priority than a first arbitration parameter of said given group.15. Arbitration circuitry as claimed in claim 14, wherein each group ofsignal lines contains N signal lines with different signal lines withineach group of signal lines corresponding to different values of saidsecond arbitration parameter, said comparison circuitry configured todischarge said different signal lines within a threshold groupassociated with a highest priority first arbitration parameter independence upon respective unique values of said second arbitrationparameter associated with said N signal inputs such that a single signalline within said threshold group remains charged thereby identifyingwhich of said N signal inputs is to be selected as said selected signalinput.
 16. A method of generating a selection signal so as to perform anarbitration in a single cycle between a plurality of said N signalinputs having respective data to send, where N is an integer of two ormore, wherein said arbitration is performed in dependence upon: (i)respective values of a first arbitration parameter associated with eachof said plurality of N signal inputs; and (ii) when two or more of saidplurality of N signal inputs have a common value of said firstarbitration parameter, a second arbitration parameter associated witheach of said two or more of plurality of N signal inputs, said secondarbitration parameter having a different value for each of said two ormore of said plurality of N signal inputs.